Expandable resin beads of styrene-modified straight-chain and low-density polyethylene, process for production thereof, pre-expand beads, and foams

ABSTRACT

According to the present invention, there is provided expandable beads of a styrene-modified linear low-density polyethylene-based resin comprising a volatile blowing agent and a base resin, the base resin containing more than 300 parts by weight and less than 1000 parts by weight of a polystyrene-based resin component relative to 100 parts by weight of a non-crosslinked linear low-density polyethylene-based resin component, wherein the base resin contains 2 to 40 wt % of a gel component comprising a graft copolymer of the polystyrene-based resin component and the low-density polyethylene-based resin component.

TECHNICAL FIELD

The present invention relates to expandable particles of astyrene-modified linear low-density polyethylene-based resin, productionmethod therefor, pre-expanded particles and expanded molded article.

BACKGROUND ART

A polyethylene-based resin foam is generally used as a packing materialowing to its high resilience and excellent oil and impact resistance.The polyethylene-based resin foam, however, has drawbacks that itsstiffness and compressive strength are low. On the other hand, apolystyrene-based resin foam is excellent in stiffness, but has adrawback that it is brittle.

To overcome such drawbacks, Japanese Examined Patent Publication No. SHO51 (1976)-46138 and Japanese Unexamined Patent Publication No. SHO 62(1987)-59642 disclose a method for obtaining expandable particles of astyrene-modified polyethylene-based resin by impregnating a styrenemonomer into a polyethylene-based resin for polymerization.

Examples of the polyethylene-based resin used in the methodsubstantially include low-density polyethylene, high-densitypolyethylene, and an ethylene-vinyl acetate copolymer. In most cases,polyethylene is cross-linked to improve its moldability and physicalproperties of a molded product. The cross-linking of polyethyleneincreases the strength of foam membrane and also increases the tensionof the membrane at expansion molding, so that the membrane is preventedfrom breaking and an expansion ratio can be increased.

Consequently, an expanded molded article with a fine appearance isprovided and the impact strength of the expanded molded article can beincreased. This method, however, requires that pre-crosslinkedpolyethylene be used or a cross-linking step be provided forcross-linking polyethylene after a temperature is furthermore raised atthe end of the polymerization of styrene monomer.

To solve the above-mentioned problems, Japanese Patent No. 2668384 alsodiscloses a method for obtaining an expanded molded article of amodified polyethylene-based resin excellent in stiffness and impactresistance. In this method, 100 parts by weight of non-crosslinkedlinear low-density polyethylene-based resin particles, 5 to 300 parts byweight of a vinyl aromatic monomer, and 1 to 3 parts by weight of apolymerization initiator relative to 100 parts by weight of the vinylaromatic monomer are dispersed in an aqueous medium. Then, a suspensionthus obtained is heated at such a temperature that polymerization of themonomer does not substantially take place for impregnation of themonomer into an inside and a surface of the polyethylene-based resinparticles. Subsequently, the temperature of the suspension is raised topolymerize the monomer, as a result, the expanded molded article of themodified polyethylene-based resin is obtained by micro-dispersion of avinyl aromatic polymer in polyethylene.

In Examples of the above-mentioned patent, a styrene monomer is added tolinear low-density polyethylene-based resin particles having a meltingpoint of 122° C. for polymerization at 115° C. (the melting point of theresin particles is not specified in Examples, but the present inventorsconfirmed from the product name of the resin particles described inExamples that the particles have the above-mentioned melting point). Thepolymerization at this temperature often results in graft polymerizationof styrene monomer on a polyethylene chain. Consequently, though aresultant resin is not cross-linked, the graft polymerization ofpolystyrene on the polyethylene chain occurs, and thereby a fineexpanded molded article can be provided. Hereinafter, the term “graftpolymer” means a gel component containing polystyrene, and the term“crosslinked polymer” means a gel component substantially not containingpolystyrene.

In the above-mentioned patent, an amount of the styrene monomer is 5 to300 parts by weight relative to 100 parts by weight of polyethylene, andwhen more than 300 parts by weight of the styrene monomer is impregnatedinto polyethylene, there is a problem that a large amount of polymerpowder of polystyrene is generated.

Where a styrene monomer is impregnated into polyethylene-based resinparticles for polymerization to obtain a resin which is subsequentlyimpregnated with a volatile blowing agent and molded by heating toobtain an expanded molded article, polyethylene needs to be cross-linkedfor enhancing impact resistance of the expanded molded article and forreducing a size variation of the expanded molded article after heating.In other words, the cross-linking of polyethylene is required for theexpanded molded article to have heat resistance and higher stiffness.However, there has been a problem that the cross-linking involves anincrease in cost due to the use of a crosslinker and an increase in aproduction step.

Accordingly, there has been desired development of expandable particlesof a styrene-modified linear low-density polyethylene-based resincapable of providing an expanded molded article having sufficientstrength, in which a ratio of styrene monomer to polyethylene can bewidely changed.

DISCLOSURE OF INVENTION

The object of the present invention is to provide an expanded moldedarticle having an improved stiffness and an excellent dimensionalstability at heating while maintaining a fundamental property, i.e.,excellent impact resistance that polyethylene possesses, withoutcross-linking a polyethylene chain.

The present inventors made an extensive study to achieve theabove-mentioned object and found that expandable particles of astyrene-modified linear low-density polyethylene-based resin areobtained by selecting a linear low-density polyethylene-based resin aspolyethylene, and by controlling selection of types and amount of apolymerization initiator and a polymerization temperature whenimpregnating a styrene-based monomer into the resin for polymerization.The expandable particles thus obtained can control an amount of a gelcomponent derived from, for example, graft polymerization of styrene onthe polyethylene chain, and provide an expanded molded article whichsatisfies physical properties such as impact resistance, stiffness andheat resistance. Thus, the present inventors achieved the presentinvention.

According to the present invention, there is provided a method forproducing expandable particles of a styrene-modified linear low-densitypolyethylene-based resin comprising, in the order recited, the steps of:

dispersing 100 parts by weight of non-crosslinked linear low-densitypolyethylene-based resin particles, 30 to 300 parts by weight of astyrene-based monomer, and 0.1 to 0.9 parts by weight of apolymerization initiator relative to 100 parts by weight of thestyrene-based monomer into a suspension containing a dispersant;

impregnating the styrene-based monomer into the low-densitypolyethylene-based resin particles by heating a resultant dispersion atsuch a temperature that polymerization of the styrene-based monomer doesnot substantially take place;

performing a first polymerization of the styrene-based monomer at atemperature of higher than (T−8)° C. and lower than (T+1)° C. (where T °C. is a melting point of the low-density polyethylene-based resinparticles);

adding a styrene-based monomer and 0.1 to 0.9 parts by weight of apolymerization initiator relative to 100 parts by weight of thestyrene-based monomer when a conversion ratio of polymerization reachesto 80 to 99.9%, and performing impregnation of the styrene-based monomerinto the polyethylene-based resin particles and a second polymerizationof the styrene-based monomer at a temperature of higher than (T−15)° C.and lower than (T+5)° C. (where T ° C. is a melting point of thepolyethylene-based resin particles) (wherein a total amount of thestyrene monomers used in the first and second polymerizations is morethan 300 parts by weight and not more than 1000 parts by weight relativeto 100 parts by weight of the low-density polyethylene-based resinparticles); and

impregnating a volatile blowing agent during or after thepolymerization,

whereby resin components of the expandable particles contain a gelcomponent comprising 2 to 40 wt % of a graft polymer.

Further, the present invention provides expandable particles of astyrene-modified linear low-density polyethylene-based resin comprisinga volatile blowing agent and a base resin, the base resin containingmore than 300 parts by weight and not more than 1000 parts by weight ofa polystyrene-based resin component relative to 100 parts by weight of anon-crosslinked linear low-density polyethylene-based resin component,wherein the base resin contains 2 to 40 wt % of a gel componentcomprising a graft polymer of the low-density polyethylene-based resincomponent and the polystyrene-based resin component.

Still further, the present invention provides pre-expanded particleshaving a bulk density of 20 to 200 kg/m³ obtained by pre-expanding theabove-mentioned expandable particles of the styrene-modified linearlow-density polyethylene-based resin.

Still yet, the present invention provides an expanded molded articlehaving a density of 20 to 200 kg/m³ thus obtained by expansion moldingof the above-mentioned pre-expanded particles.

BEST MODE FOR CARRYING OUT THE INVENTION

Expandable particles of a styrene-modified linear low-densitypolyethylene-based resin (hereinafter referred to as expandable resinparticles) obtained by the production method of the present inventioncomprise a volatile blowing agent and a base resin containing anon-crosslinked linear low-density polyethylene-based resin componentand the polystyrene-based resin component.

As the non-crosslinked linear low-density polyethylene-based resincomponent (hereinafter referred simply to as a polyethylene-based resincomponent) used in the present invention, a copolymer of ethylene and anα-olefin can be mentioned.

Examples of the α-olefin include 1-butene, 1-pentene, 1-hexene,3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, and1-octene. Among those, 1-butene and 1-hexene are preferable.

A ratio of ethylene to the α-olefin may vary depending upon physicalproperties desired and preferably in a range of 1:0.01 to 1:0.1 (weightratio). The term “low-density” means a density in a range of 0.910 to0.925 g/ml.

Low-density polyethylene, high-density polyethylene, anethylene-propylene copolymer, an ethylene-vinyl acetate copolymer and anethylene-acrylic acid copolymer which have a cross-link and/or abranched chain, and two or more types of these polymers may be used tosuch an extent that a desirable effect of the invention is notsuppressed.

Examples of the polystyrene-based resin component include resincomponents derived from monomers such as styrene, α-methylstyrene,vinyltoluene, and chlorostyrene.

An amount of the polystyrene-based resin component is more than 300parts by weight and not more than 1000 parts by weight, and preferablymore than 300 parts by weight and not more than 900 parts by weightrelative to 100 parts by weight of the polyethylene-based resincomponent.

While it is difficult to obtain the expandable resin particles uniformlycontaining not less than 300 parts by weight of the polystyrene resincomponent by conventional methods, the present invention can easilyobtain the particles in a proper manner. In the case where the amount ofpolystyrene-based resin component exceeds 1000 parts by weight,characteristics of the polyethylene-based resin component, i.e., highresilience and excellent oil and impact resistance are hardly displayed.Furthermore, since styrene is not sufficiently absorbed into an insideof the polyethylene-based resin component and is polymerized alone, alarge amount of polymer powder is generated.

As the volatile blowing agent, for example, a hydrocarbon such aspropane, butane, isobutene, pentane, isopentane, cyclopentane, andhexane may be used alone, or two or more types of these hydrocarbons maybe used in combination.

A content of the blowing agent is preferably 5 to 10 parts by weightrelative to 100 parts by weight of the resin component (an amount of thepolyethylene-based resin component and the polystyrene-based resincomponent in total) which constitute the expandable resin particles.

In the present invention, 2 to 40 wt % of the gel component (gelfraction) comprising the graft polymer of the polyethylene-based resincomponent and the polystyrene-based resin component is contained in thebase resin of the expandable resin particles. A criterion for judgingwhether or not the gel component is a graft polymer is the presence orabsence of polystyrene in the gel component. In the present invention,the gel component containing 10 wt % or more of polystyrene is definedas the graft polymer. A method for determining a polystyrene content inthe gel component is described in Examples.

A gel fraction of less than 2 wt % is not preferable since changes indimension of the expanded molded article at heating and the impactresistance are not sufficiently improved. On the other hand, the gelfraction of more than 40 wt % is also not preferable since the strengthof cell membranes is too high and elongation at formation of theexpanded molded article is poor, resulting in deterioration of theexpanded molded article in appearance. Thus, the gel fraction ispreferably 5 to 35 wt %.

Preferably, the expandable resin particles each have a substantiallyspherical shape or a cylindrical shape having an L/D (where L is alength of each particle and D is a diameter of each particle) of 0.6 to1.6, and an average particle size of 0.3 to 2.0 mm.

The particles having a high ovality such that the L/D is not more than0.6 and not less than 1.6 are not preferable since the expandable resinparticles have poor mold fillability when they are pre-expanded asexpandable styrene-modified resin particles and filled into a mold toobtain the expanded molded article. The shape of the expandable resinparticles preferably is substantially spherical to improve the moldfillability.

The average particle size of less than 0.3 mm is not preferable becausea retention of the blowing agent decreases and a reduction of density ofthe expanded molded article is difficult. The average particle size ofmore than 2.0 mm is also not preferable since not only the particleshave poor mold fillability, but also thinning of the expanded moldedarticle is difficult.

A production method for the expandable resin particles of the inventionwill hereinafter be described.

Where the amount of styrene-based monomer exceeds 300 parts by weightrelative to 100 parts by weight of the polyethylene-based resinparticles, the amount of polymer powder of polystyrene tends to increasewhen only a single impregnation is performed. To minimize the generationof the polymer powder, the styrene-based monomers are impregnated intothe polyethylene-based resin particles in two separate steps asdescribed below.

Preferably, the polyethylene-based resin particles used in the methodeach have the substantially spherical shape or the cylindrical shapehaving an L/D (where L is a length of each particle and D is a diameterof each particle) of 0.6 to 1.6, and preferably have an average particlesize of 0.2 to 1.5 mm. The polyethylene-based resin particles having ahigh ovality such that the L/D is not more than 0.6 and not less than1.6 are not preferable since the polyethylene-based resin particles havepoor mold fillability when they are pre-expanded as the expandablestyrene-modified resin particles and filled into a mold to obtain theexpanded molded article. The shape of the polyethylene-based resinparticles preferably is substantially spherical to improve moldfillability. The average particle size of less than 0.2 mm is notpreferable since a retention of the blowing agent decreases and areduction of the density of the expanded molded article is difficult.The average particle size of more than 1.5 mm is also not preferablebecause not only the particles have poor mold fillability, but alsothinning of the expanded molded article is difficult.

Examples of an aqueous medium which is a component of the suspensioninclude water and a mixed medium of water and an aqueous solvent (e.g.,a lower alcohol).

A dispersant is not particularly limited and any conventional dispersantmay be used. Specifically, the dispersant may be practically insolubleinorganic substances such as calcium phosphate, magnesium pyrophosphate,sodium pyrophosphate, and magnesium oxide.

A polymerization initiator may be one that is generally used as asuspension polymerization initiator of the styrene-based monomer.However, a gel generation rate varies depending upon a type of thepolymerization initiator to be used. For example, use of t-butylperoxybenzoate, dicumyl peroxide, t-butyl-peroxy-2-ethylhexyl carbonate,2,5-dimethyl-2,5-di-t-butylperoxyhexane or the like which abstracts morehydrogen atoms increases the amount of gel to be generated. On the otherhand, use of t-butyl peroxy-3,5,5-trimethylhexanoate, t-butylperoxyacetate, 2,2-di-t-butyl peroxybutane or the like which abstractsless hydrogen atoms decreases the amount of gel to be generated. Theabove-mentioned polymerization initiators may be adopted alone, or twoor more types of these polymerization initiators may be used incombination depending upon a desirable gel fraction.

An amount of the polymerization initiator is preferably 0.1 to 0.9 partsby weight, and more preferably 0.2 to 0.5 parts by weight relative to100 parts by weight of the styrene-based monomer. The polymerizationinitiator in an amount of less than 0.1 parts by weight is notpreferable since styrene is not smoothly polymerized, and therebypolystyrene and polyethylene are not homogeneously mixed in the resinparticles and a larger amount of polymer powder is generated. Use of thepolymerization initiator in an amount of more than 0.9 parts by weightdecreases a molecular weight of the polystyrene-based resin.

A molecular weight of polystyrene-based resin component is preferablyabout 200,000 to 400,000 to achieve fine physical properties. Thepolymerization initiator in an amount of more than 0.9 parts by weightcan only achieve a styrene molecular weight less than the above range.

An amount of the styrene-based monomer used in the first polymerizationis in a range of 30 to 300 parts by weight relative to 100 parts byweight of the polyethylene-based resin particles. The styrene-basedmonomer in an amount of less than 30 parts by weight is not preferablesince the styrene-based monomer is not homogeneously impregnated intopolyethylene. The styrene-based monomer in an amount of more than 300parts by weight is not preferable since the polymer powder derived fromthe styrene-based monomer is more easily generated.

The resultant dispersion is heated to such a temperature thatpolymerization of the styrene-based monomer does not substantially takeplace to impregnate the styrene-based monomer into thepolyethylene-based resin particles.

An appropriate time required for impregnating the styrene-based monomerinto the inside of the polyethylene-based resin particles is 30 minutesto 2 hours, since when the polymerization proceeds before thestyrene-based monomer is sufficiently impregnated into the resinparticles, the polymer powder of polystyrene is generated. Thetemperature at which the styrene-based monomer is substantially notpolymerized should be higher to accelerate an impregnation rate, but thetemperature needs to be determined in due consideration of adecomposition temperature of the polymerization initiator.

The first polymerization of the styrene-based monomer is carried out ata temperature of (T−8) to (T+1)° C. (where T ° C. is the melting pointof the above-mentioned polyethylene-based resin particles). Where thepolymerization temperature is lower than (T−8)° C. or higher than (T+1)°C., sufficient graft polymerization can not be achieved.

When the conversion ratio of polymerization reaches to 80 to 99.9%, thestyrene-based monomer and 0.1 to 0.9 parts by weight of thepolymerization initiator relative to 100 parts by weight of thestyrene-based monomer are added, and the temperature is set to (T−15) to(T+5)° C. (where T ° C. is the melting point of the polyethylene-basedresin particles), so that the impregnation of the styrene-based monomerinto the low-density polyethylene-based resin particles and a secondpolymerization are performed. The amount of the styrene-based monomerused in the first and second polymerization in total is more than 300parts by weight and not more than 1000 parts by weight.

Where more than 300 parts by weight of the styrene-based monomer isadded when the conversion ratio of polymerization reaches to 80%, thestyrene-based monomer is quickly impregnated into the polyethylene-basedresin particles for quick polymerization, whereby the generation ofpolymer powder of polystyrene can be suppressed. When the conversionratio of polymerization exceeds 99.9%, the impregnation of thestyrene-based monomer added is difficult and a conversion ratiodecreases, causing the generation of the powdered particles difficult tobe suppressed.

A mixed solution containing the styrene-based monomer and thepolymerization initiator may be added continuously or intermittently forthe second polymerization. To prevent the generation of the polymerpowder, the impregnation into the inside of the polyethylene-based resinparticles and the polymerization are preferably performed at almost thesame time. A fast addition rate is not preferable because the fastaddition rate in combination with a relatively high polymerizationtemperature causes the polymerization to occur before the resinparticles are impregnated. An extremely slow addition rate is notpreferable since it may hinder the polymerization. For example, theaddition rate for 300 to 1000 parts by weight of the styrene-basedmonomer is 3 to 5 hours.

Furthermore, the polymerization temperature of the second polymerizationpreferably ranges from (T−8) to (T+1)° C., so that the graftpolymerization occurs more efficiently.

The volatile blowing agent is impregnated into the resin particlesduring or after the polymerization, so that the expandable resinparticles are obtained. This impregnation of the blowing agent can beperformed by a per se known method. For example, the impregnation duringpolymerization can be carried out by conducting the polymerization in aclosed vessel, and by injecting the volatile blowing agent into thevessel. The impregnation after the polymerization is carried out byinjecting the volatile blowing agent into a closed vessel. In theimpregnation after the polymerization, a vessel for the polymerizationmay not be a closed vessel as long as the impregnation is carried out inthe closed vessel.

The expandable resin particles of the present invention thus obtainedmay serve as pre-expanded particles by pre-expanding the expandableresin particles to a predetermined bulk density (e.g., 20 to 200 kg/m³)by a conventional method. A method for determining the bulk density willbe described in Examples.

Furthermore, an expanded molded article is provided by filling thepre-expanded particles into a mold and heating the pre-expandedparticles again to allow them to fuse together by heat while expansion.

As a medium for heating the pre-expanded particles, steams are suitablyused. A density of the expanded molded article is preferably 20 to 200kg/m³. The expanded molded article having a density of less than 20kg/m³ is not preferable since sufficient strength is not achieved. Theexpanded molded article having a density of more than 200 kg/m³ is alsonot preferable since a reduction in weight can not be achieved and theexpanded molded article may not be able to sufficiently exertcharacteristics of polyethylene including resilience.

The expanded molded article thus obtained is tough and excellent inimpact strength. Furthermore, the expanded molded article has highstiffness due to the modification of styrene.

A falling ball impact value of the expanded molded article is preferably85 cm or more. The expanded molded article having a falling ball impactvalue of less than 85 cm may be used, but if the expanded molded articlehas a falling ball impact value of not less than 85 cm, it is applicableto packing materials, etc. for fragile products and heavy components.The falling ball impact value of the expanded molded article is morepreferably 90 cm or more. A method for measuring the falling ball impactvalue will be described in Examples.

The expanded molded article of the present invention may be used forvarious purposes, and particularly suitable for material for interiorfurnishings of a car, energy absorbing material to be inserted inside abumper, packing material for heavy products, and the like.

EXAMPLES

The present invention will hereunder be described with reference toExamples and Comparative Examples, but it should be understood that theinvention be not limited by these Examples and Comparative Examples.Methods for determining values shown in Examples and ComparativeExamples are described below.

(Determination of Gel Fraction)

For determination of a gel fraction, a sample of resin particles wasweighed, the sample was put in a flask, and 100 ml of toluene was added.The particles were then dissolved in an oil bath at 130° C. for 24hours. After the flask was taken out from the oil bath, a resultantmixture was immediately filtered by an 80-mesh (φ0.12 mm) wire gauze,and a sample remaining on the wire gauze which is insoluble in boilingtoluene and the wire gauze were then allowed to stand in an oven at 130°C. for an hour to remove toluene, and a weight of the resultant solidwas measured. The gel fraction is determined by the following equation:

${\frac{\text{Weight~~of~~resultant~~solid}}{\text{Weight~~of~~sample}} \times 100} = \text{gel~~fraction~~(wt~~\%)}$

About 200 μg of the resultant solid was weighed and enwrapped in aferromagnetic metal (Pyrofoil: manufactured by Japan Analytical IndustryCo., Ltd.) so as to be in close contact with each other. Then, apyrolysate was generated using a pyrolysis apparatus called Curie PointPyrolyzer Model JHP-3 (manufactured by Japan Analytical Industry Co.,Ltd.). The pyrolysate was analyzed using Gas Chromatograph Auto System(manufactured by Perkin Elmer) to determine an polystyrene content fromthe analytical result. The following analytical conditions were adopted.Pyrolysis temperature: 590° C.-5 sets, oven temperature: 280° C., needletemperature: 300° C., column: DB-5 (0.25 μm×φ0.25 mm×30 m, manufacturedby J & W), column temperature: 50° C. (1 min)→temperature rise of 10°C./min→100° C. temperature rise of 40° C./min→320° C. (3.5 min), carriergas: He, carrier flow rate: 1 ml/min, pressure at column inlet: 12 psi,temperature at column inlet: 300° C., temperature of a detector: 300°C., and detector: FID. Determination was made by an absolute calibrationcurve method using polystyrene resin QC254 manufactured by Asahi KaseiCo., as a standard sample.

When the polystyrene content is 10 wt % or more, it was determined thata gel component comprises a graft polymer but not a cross-linkedpolymer.

(Measurement of Powder Content)

For measurement of a powder content, about 1000 g of a polymerizedslurry sample was introduced into a polymer beaker having a water ventwith 35-mesh wire gauze attached on its top. Into this beaker about 6liter of wash water was gradually introduced, and liquid flowed out ofthe vent was collected. The collected liquid was filtered by a glassfiber paper filter (GA-100) and dried in an oven at 60° C. for 3 hoursto measure the weight of dried polymer powder. The resin remained in theslurry sample after washing was dried and weighed as well. The powdercontent was determined by the following equation:

${\frac{\text{Weight~~of~~powder~~resin~~(g)}}{\text{Weight~~of~~dried~~resin~~(g)}} \times 100} = \text{powder~~content~~(wt~~\%)}$

(Measurement of Molecular Weight of Polystyrene Resin Component in ResinParticles)

An average molecular weight (Mw) of a polymer was measured by GPC (GelPermeation Chromatography) under the following conditions.

Measuring Equipment:

-   -   High-speed GPC equipment HLC-8020 manufactured by Tosoh Corp.        Column: HSG-60S×2, HSG-40H×1, HSG-20H×1 manufactured by Sekisui        Fine Chemicals Co., Ltd.        Measuring Conditions:    -   Column temperature: 40° C.    -   Moving bed: THF (tetrahydrofuran)    -   Flow rate: 1.0 ml/min    -   Injection amount: 500 ml    -   Detector: RID-6A    -   manufactured by Tosoh Corp.

Molecular weight determination of the sample: For measuring a molecularweight of a sample, conditions for measuring were selected so that amolecular weight distribution of the sample overlaps a range of a linearcalibration curve correlating a count number with a logarithm of themolecular weight of various monodisperse polystyrene standard samples.

In the present invention, the calibration curve for polystyrene wasplotted using six polystyrene standard samples (TSK standardpolystyrene) respectively having a weight-average molecular weight of2.74×10³, 1.91×10⁴, 1.02×10⁵, 3.55×10⁵, 2.89×10⁶, 4.48×10⁶ manufacturedby Tosoh Corp.

(Determination of Bulk Density)

A bulk density was determined according to a method described in JIS K6911:1955 “General Testing Methods for Thermosetting Plastics”.Specifically, pre-expanded particles free-falling into a graduatedcylinder by an apparent density measuring instrument was weighed todetermine their bulk density by the following equation.bulk density (kg/m³)=weight (kg)/bead volume in the graduated cylinder(m³)

(Determination of Density of Expanded Molded Article)

A density of an expanded molded article was determined according to amethod described in JIS A 9511:1995 “Preformed Cellular Plastics ThermalInsulation Materials”.

(Measurement of Compressive Strength)

A compressive strength was measured according to a method described inJIS A 9511:1995 “Preformed Cellular Plastics Thermal InsulationMaterials”. In other words, the compressive strength of a test specimenhaving a size of 50×50×50 mm was measured when it was compressed by 5%using a universal testing machine Tensilon UCT-10T (manufactured byOrientech Co.) under a compressive rate of 10 mm/min.

(Measurement of Impact Strength)

For measurement of impact strength, an expanded molded article was cutto form a sample having a size of 215×40×20 mm, and the sample was thenplaced on a pair of holding members arranged at a distance of 155 mm. Asteel ball weighing 321 g was added on the middle of the sample in widthdirection thereof at a position halfway between the pair of holdingmembers to see whether or not the sample was crushed.

The test was repeated at different heights of drop and the minimumheight of drop that produced crush on the sample was defined as afalling ball impact value to evaluate the impact strength. Thus, theimpact strength increases as the falling ball impact value increases.

(Determination of Dimensional Change Rate Under Heat)

A flat-shaped expanded molded article having a size of 400 mm length,300 mm width, and 16 mm thickness was taken out from a mold, and allowedto stand at a constant temperature for 24 hours in a thermohygrostat (ina state of standard temperature of 23° C. and 50% relative humidity ofJIS-K7100). Then, a flat square board (150 mm length, 150 mm width, and16 mm thickness) having parallel, upper and lower surfaces was cut outfrom the center of the expanded molded article. On the middle of theboard, three straight parallel lines were drawn respectively in verticaland lateral direction so as to be 50 mm apart from each other to form aspecimen that complies with JIS-K6767. After the specimen was measuredfor its dimension (dimension before heating: L1), it was placedhorizontally in an oven with internal air circulation maintained at 80°C. The specimen was then taken out from the oven after being heated for168 hours, and again allowed to stand at the constant temperature for anhour in the thermohygrostat to measure the specimen for its dimension(dimension after heating: L2). The measurement of dimensions before andafter the heating test was carried out in compliance with JIS-K6767, anda dimensional change rate was determined by the following equation.dimensional change rate (%)=(L2−L1)×100/L1(wherein L1 is a dimension of the specimen obtained from the expandedmolded article after it was molded and kept standing at 23° C. and 50%RH for 24 hours. L2 is a dimension of the specimen after the expandedmolded article was heated at 80° C. for 168 hours.)

The dimension of the specimen is an average length of the three linesprovided both in vertical and horizontal directions of the specimenobtained from the expanded molded article.

It was determined that the expanded molded article with a dimensionalchange rate of 0.5% or less has heat resistance.

Example 1

(Production Of Polyethylene-Based Resin Particles)

Linear low-density polyethylene (ethylene-hexene copolymer, melt indexof 1.0 g/10 min, density of 0.921 g/ml, melting point of 126° C.) wasgranulated by an extruder to obtain polyethylene-based resin particlesof substantially spherical shape having an L/D of 0.9 and an averageparticle size of 0.8 mm. As a foam regulator, 0.5 parts by weight oftalc relative to 100 parts by weight of the above-mentioned polyethylenewas added at the granulation.

(Production of Styrene-Modified Polyethylene-Based Resin Particles)

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8 kg of the above-mentioned polyethylene-based resin particleswas suspended in the aqueous medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 16 kg of a styrenemonomer (200 parts by weight relative to 100 parts by weight ofpolyethylene) and 48 g of t-butylperoxy-2-ethylhexylcarbonate (TBPOEHC)(0.3 parts by weight relative to 100 parts by weight of the styrenemonomer) as a polymerization initiator was added and allowed to stand at60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin.

Then, the temperature was raised to 119° C. and polymerization wascarried out for 3 hours. After a conversion ratio of polymerization ofthe styrene monomer reached to 85%, a mixed solution containing 16 kg ofthe styrene monomer (200 parts by weight relative to 100 parts by weightof polyethylene) and 48 g of t-butylperoxybenzoate (TBPB) (0.3 parts byweight relative to 100 parts by weight of the styrene monomer) as apolymerization initiator was added over a period of 3 hours to performpolymerization while impregnating the styrene monomer into the inside ofpolyethylene. Then, by raising to 140° C. and maintained for 2 hours,the remaining monomer was forcibly polymerized to reduce its amount, andthen the autoclave was cooled to take out styrene-modifiedpolyethylene-based resin particles.

The gel fraction measured was 7.2 wt %. The polystyrene content in thegel component was 22.2 wt % and the powder content in the polymerizedslurry was 0.7 wt %. The molecular weight of the polystyrene resincomponent was about 320,000.

(Production of Expandable Particles of Styrene-ModifiedPolyethylene-Based Resin and Evaluations of their Expandability andMoldability)

20 kg of the above-mentioned styrene-modified polyethylene-based resinparticles and 400 g of toluene were introduced into a pressure-resistanttwin-cylinder mixer which has an internal volume of 50 liter and can behermetically sealed. After the mixer was hermetically sealed, it wasrotated and 2800 g of butane (n-butane: i-butane=7:3, volume ratio,butane with the same volume ratio was used in the following Examples)was forced into the mixer. Then, the temperature was raised to 70° C.and maintained for 4 hours to impregnate butane into the particles. Themixer was then cooled and expandable particles of a styrene-modifiedpolyethylene-based resin were taken out.

The expandable resin particles taken out from the mixer were immediatelypre-expanded by steam to have a bulk density of 33 kg/m³. About 24 hourslater, the pre-expanded resin particles were filled into a mold andheated by steam to allow them to fuse together while being pre-expanded,so that an expanded molded article having a density of 33 kg/m³ areobtained. The expanded molded article thus obtained had excellentstrength, showing a compressive strength of 40 N/cm² and falling ballimpact value as high as 85 cm. The dimensional change rate was 0.4%.

Example 2

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 121° C., to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The pre-expanded resin particles had a bulk density of 33 kg/m³. The gelfraction measured was 26.5 wt %. The polystyrene content in the gelcomponent was 31.9 wt %, and the powder content in the polymerizedslurry was 0.8 wt %. The molecular weight of the polystyrene resincomponent was about 310,000.

The expanded molded article thus obtained had excellent strength,showing a density of 33 kg/m³, compressive strength of 42 N/cm², andfalling ball impact value as high as 95 cm. The dimensional change rateunder heat was 0.3%.

Example 3

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 122° C., to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction measured was 33.5 wt %. The polystyrene content in thegel component was 20.9 wt % and the powder content in the polymerizedslurry was 0.7 wt %. The molecular weight of the polystyrene resincomponent was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 44 N/cm², and falling ballimpact value as high as 95 cm. The dimensional change rate under heatwas 0.2%.

Example 4

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 123° C., to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction measured was 20.8 wt %. The polystyrene content in thegel component was 22.1 wt % and the powder content in the polymerizedslurry was 0.6 wt %. The molecular weight of the polystyrene resincomponent was about 290,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 42 N/cm², and falling ballimpact value as high as 90 cm. The dimensional change rate under heatwas 0.3%.

Example 5

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 125° C., to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction measured was 5.8 wt %. The polystyrene content in thegel component was 34.5 wt % and the powder content in the polymerizedslurry was 0.5 wt %. The molecular weight of the polystyrene resincomponent was about 280,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 40 N/cm², and falling ballimpact value as high as 85 cm. The dimensional change rate under heatwas 0.4%.

Example 6

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 4.4 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 13.2 kg of a styrenemonomer (300 parts by weight relative to 100 parts by weight ofpolyethylene) and 39.6 g of TBPOEHC (0.3 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin.

Then, the temperature was raised to 122° C. and polymerization wascarried out for 3 hours. After the conversion ratio of polymerization ofthe styrene monomer reached to 85%, a mixed solution containing 22.4 kgof a styrene monomer (500 parts by weight relative to 100 parts byweight of polyethylene) and 67.2 g of TBPB (0.3 parts by weight relativeto 100 parts by weight of the styrene monomer) was added at 122° C. overa period of 5 hours to perform polymerization while impregnating thestyrene monomer into the inside of polyethylene. Then, by raising to140° C. and maintained for 2 hours, the remaining monomer was forciblypolymerized to reduce its amount, and then the autoclave was cooled totake out styrene-modified polyethylene-based resin particles.

The gel fraction measured was 38.5 wt %. The polystyrene content in thegel component was 21.3 wt % and the powder content in the polymerizedslurry was 0.8 wt %. The molecular weight of the polystyrene resincomponent was about 300,000.

The same procedure as in Example 1 was repeated except that the obtainedresin particles were used, so that a pre-expanded resin particles havinga bulk density of 33 kg/m³ and an expanded molded article having adensity of 33 kg/m³ were obtained. The expanded molded article thusobtained had excellent strength, showing a compressive strength of 48N/cm² and falling ball impact value as high as 85 cm. The dimensionalchange rate under heat was 0.2%.

Example 7

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 8.0 kg of a styrenemonomer (100 parts by weight relative to 100 parts by weight ofpolyethylene) and 24.0 g of TBPB (0.3 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to119° C. and polymerization was carried out for 3 hours. After theconversion ratio of polymerization of the styrene monomer reached to85%, a mixed solution containing 24.0 kg of a styrene monomer (300 partsby weight relative to 100 parts by weight of polyethylene), and 27.0 gof TBPB (0.3 parts by weight relative to 100 parts by weight of thestyrene monomer) was added at 121° C. over a period of 4 hours toperform polymerization while impregnating styrene into the inside ofpolyethylene. Then, by raising to 140° C. and maintained for 2 hours,the remaining monomer was forcibly polymerized to reduce its amount, andthen the autoclave was cooled to obtain styrene-modifiedpolyethylene-based resin particles. Pre-expanded resin particles and anexpanded molded article were subsequently obtained in the same manner asin Example 1.

The gel fraction in the resin particles thus obtained was 7.2 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent, in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 43 N/cm², and falling ballimpact value as high as 85 cm. The dimensional change rate under heatwas 0.4%.

Example 8

The same polymerization procedure as in Example 7 was repeated exceptthat the temperature for the first polymerization was 126° C. and thatDCP was used as the polymerization initiator for the firstpolymerization, to obtain styrene-modified polyethylene-based resinparticles, pre-expanded resin particles and an expanded molded articlethereof.

The gel fraction in the resin particles thus obtained was 3.5 wt %. Thepolystyrene content in the gel component was 22.8 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 42 N/cm², and falling ballimpact value as high as 85 cm. The dimensional change rate under heatwas 0.4%.

Example 9

The same polymerization procedure as in Example 7 was repeated exceptthat the temperatures for the first and second polymerization were 123°C. and 112° C., respectively, and thatt-butylperoxy-3,5,5-trimethylhexanoate (TBPOTMH) was used as thepolymerization initiator for the second polymerization, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction in the resin particles thus obtained was 3.2 wt %. Thepolystyrene content in the gel component was 24.4 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 340,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 43 N/cm², and falling ballimpact value as high as 85 cm. The dimensional change rate under heatwas 0.3%.

Example 10

The same polymerization procedure as in Example 7 was repeated exceptthat the temperatures for the first and second polymerization were 123°C. and 130° C., respectively, and that DCP was used as thepolymerization initiator for the first polymerization, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction in the resin particles thus obtained was 3.8 wt %. Thepolystyrene content in the gel component was 24.2 wt % and the powdercontent in the polymerized slurry was 0.6 wt %. The molecular weight ofthe polystyrene resin component was about 240,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 43 N/cm², and falling ballimpact value as high as 85 cm. The dimensional change rate under heatwas 0.4%.

Example 11

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 8.0 kg of a styrenemonomer (100 parts by weight relative to 100 parts by weight ofpolyethylene) and 48.0 g of TBPB (0.6 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to120° C. and polymerization was carried out for 3 hours.

After the conversion ratio of polymerization of the styrene monomerreached to 90%, a mixed solution containing 24.0 kg of a styrene monomer(300 parts by weight relative to 100 parts by weight of polyethylene)and 144.0 g of TBPB (0.6 parts by weight relative to 100 parts by weightof the styrene monomer) was added at 120° C. over a period of 4 hours toperform polymerization while impregnating styrene into the inside ofpolyethylene. Then, by raising to 140° C. and maintained for 2 hours,the remaining monomer was forcibly polymerized to reduce its amount, andthen the autoclave was cooled to obtain styrene-modifiedpolyethylene-based resin particles. Pre-expanded resin particles and anexpanded molded article were subsequently obtained in the same manner asin Example 1.

The gel fraction in the resin particles thus obtained was 19.6 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent in the polymerized slurry was 0.7 wt %. The molecular weight ofthe polystyrene resin component was about 260,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 40 N/cm², and falling ballimpact value as high as 90 cm. The dimensional change rate under heatwas 0.3%.

Example 12

The same polymerization procedure as in Example 11 was repeated exceptthat the respective amount of the polymerization initiators for thefirst and second polymerization was 0.3 parts by weight and that themixed solution for the second polymerization was added after theconversion ratio of polymerization reached to 95%, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction measured was 18 wt %. The polystyrene content in thegel component was 23.3 wt % and the powder content in the polymerizedslurry was 0.3 wt %. The molecular weight of the polystyrene resincomponent was about 320,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 46 N/cm², and falling ballimpact value as high as 95 cm. The dimensional change rate under heatwas 0.2%.

Example 13

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 8.0 kg of a styrenemonomer (100 parts by weight relative to 100 parts by weight ofpolyethylene) and 19.20 g of TBPB (0.24 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to120° C. and polymerization was carried out for 3 hours. After theconversion ratio of polymerization of the styrene monomer reached to90%, a mixed solution containing 24.0 kg of a styrene monomer (300 partsby weight relative to 100 parts by weight of polyethylene) and 57.6 g ofTBPB (0.24 parts by weight relative to 100 parts by weight of thestyrene monomer) was added at 120° C. over a period of 4 hours toperform polymerization while impregnating styrene into the inside ofpolyethylene. Then, by raising to 140° C. and maintained for 2 hours,the remaining monomer was forcibly polymerized to reduce its amount, andthen the autoclave was cooled to obtain styrene-modifiedpolyethylene-based resin particles. Pre-expanded resin particles and anexpanded molded article were subsequently obtained in the same manner asin Example 1.

The gel fraction in the resin particles thus obtained was 17.5 wt %. Thepolystyrene content in the gel component was 24.6 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 320,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 46 N/cm², and falling ball impact value of 95cm. The dimensional change rate under heat was 0.2%.

Comparative Example 1

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 117° C., and thestyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof were obtained.

The gel fraction measured was 0.3 wt %. The polystyrene content in thegel component was 26.6 wt % and the powder content in the polymerizedslurry was 0.5 wt %. The molecular weight of the polystyrene resincomponent was about 330,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 36 N/cm², and falling ball impact value of 80cm. The dimensional change rate under heat was 0.8%.

Comparative Example 2

The same polymerization procedure as in Example 1 was repeated exceptthat the temperature for polymerization was 130° C. and dicumyl peroxide(DCP) was adopted as the polymerization initiators, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction measured was 0.8 wt %. The polystyrene content in thegel component was 25.0 wt % and the powder content in the polymerizedslurry was 0.8 wt %. The molecular weight of the polystyrene resincomponent was about 240,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 38 N/cm², and falling ballimpact value as high as 80 cm. However, the dimensional change rateunder heat was 0.7%.

Comparative Example 3

The same polymerization procedure as in Example 1 was repeated exceptthat an additional styrene monomer was added when the conversion ratioof polymerization reached to 60%, to obtain styrene-modifiedpolyethylene-based resin particles, pre-expanded resin particles and anexpanded molded article thereof.

The gel fraction measured was 5.6 wt %. The polystyrene content in thegel component was 21.4 wt % and the powder content in the polymerizedslurry was 1.6 wt %. The molecular weight of the polystyrene resincomponent was about 320,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³. Sincethe polymerization did not occur smoothly, a large amount of polymerpowder of polystyrene was generated, whereby fusibility of expandedparticles was decreased. Consequently, the compressive strength was 38N/cm², and falling ball impact value was 60 cm. The dimensional changerate under heat was 0.8%.

Comparative Example 4

The same polymerization procedure as in Example 7 was repeated exceptthat the temperatures for the first and second polymerization were 117°C. and 121° C., respectively, to obtain styrene-modifiedpolyethylene-based resin particles, pre-expanded resin particles and anexpanded molded article thereof.

The gel fraction in the resin particles thus obtained was 1.6 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 38 N/cm², and falling ball impact value of 80cm. The dimensional change rate under heat was 0.7%.

Comparative Example 5

The same polymerization procedure as in Example 7 was repeated exceptthat the temperature for the first polymerization was 128° C. and thatDCP was adopted as the polymerization initiator for the firstpolymerization, to obtain styrene-modified polyethylene-based resinparticles, pre-expanded resin particles and an expanded molded articlethereof.

The gel fraction in the resin particles thus obtained was 1.8 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent in the polymerized slurry was 0.6 wt %. The molecular weight ofthe polystyrene resin component was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 40 N/cm², and falling ball impact value of 80cm. The dimensional change rate under heat was 0.7%.

Comparative Example 6

The same polymerization procedure as in Example 7 was repeated exceptthat the temperatures for the first and second polymerization were 123°C. and 110° C., respectively, and that TBPOTMH was adopted as thepolymerization initiator for the second polymerization, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction in the resin particles thus obtained was 2.6 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 360,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 37 N/cm², and falling ball impact value of 50cm. The dimensional change rate under heat was 0.4%.

Comparative Example 7

The same polymerization procedure as in Example 7 was repeated exceptthat the temperatures for the first and second polymerization were 123°C. and 132° C., respectively, and that DCP was adopted as thepolymerization initiator for the second polymerization, to obtainstyrene-modified polyethylene-based resin particles, pre-expanded resinparticles and an expanded molded article thereof.

The gel fraction in the resin particles thus obtained was 1.8 wt %. Thepolystyrene content in the gel component was 24.4 wt % and the powdercontent in the polymerized slurry was 0.6 wt %. The molecular weight ofthe polystyrene resin component was about 220,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³, acompressive strength of 40 N/cm², and falling ball impact value of 80cm. The dimensional change rate under heat was 0.7%.

Comparative Example 8

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 32.0 kg of a styrenemonomer (400 parts by weight relative to 100 parts by weight ofpolyethylene) and 96 g of TBPB (0.3 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to121° C. and polymerization was carried out for 5 hours. Subsequently, byraising to 140° C. and maintained for 2 hours, the remaining monomer wasforcibly polymerized to reduce its amount, and then the autoclave wascooled to obtain styrene-modified polyethylene-based resin particles.

The gel fraction measured was 1.7 wt %. The polystyrene content in thegel component was 20.6 wt % and the powder content in the polymerizedslurry was 1.8 wt %. The molecular weight of the polystyrene resincomponent was about 320,000.

Since the polymerized slurry contained a large amount of polymer powderand the powder impairs the fusibility of the expandable particles, finepre-expanded resin particles and an expanded molded article could not beobtained.

Comparative Example 9

The same polymerization procedure as in Example 11 was repeated exceptthat the respective amount of the polymerization initiator for the firstand second polymerization was 0.3 parts by weight and that the mixedsolution for the second polymerization was added after the conversionratio of polymerization reached to 75%, to obtain styrene-modifiedpolyethylene-based resin particles, pre-expanded resin particles and anexpanded molded article thereof.

The gel fraction measured was 7.2 wt %. The polystyrene content in thegel component was 22.2 wt %. Since the excess styrene monomer waspolymerized alone or on the surface of the polyethylene-based resinparticles, the amount of the polymer powder generated was extremelylarge such that the powder content in the polymerized slurry was 1.9 wt%. The molecular weight of the polystyrene resin component was about320,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had a density of 33 kg/m³ and acompressive strength of 45 N/cm². Since a large amount of polymer powderof polystyrene was generated and thereby fusibility of the expandedparticles decreased, the falling ball impact value was as low as 40 cm.The dimensional change rate under heat was 0.4%.

A ratio between materials and polymerization conditions of Examples 1 to13 and Comparative examples 1 to 9 are shown in Table 1. The gelfraction, polystyrene content, powder content, molecular weight of thepolystyrene resin component, compressive strength, falling ball impactvalue and dimensional change rate under heat are together shown in Table2.

TABLE 1 Polymeri- Conversion zation Polymeri- Amount of Ratio of Temp.zation Initiator PE/PS Polymeri- (1^(st)/2^(nd)) Initiator(1^(st)/2^(nd)) (1^(st)& 2^(nd)) zation (%) (° C.) (1^(st)/2^(nd)) (wt%) EXAMPLES 1 100/400(200 + 200) 85 119/119 TPOEHC/TBPB 0.3/0.3 2100/400(200 + 200) 85 121/121 TPOEHC/TBPB 0.3/0.3 3 100/400(200 + 200)85 122/122 TPOEHC/TBPB 0.3/0.3 4 100/400(200 + 200) 85 123/123TPOEHC/TBPB 0.3/0.3 5 100/400(200 + 200) 85 125/125 TPOEHC/TBPB 0.3/0.36 100/800(300 + 500) 85 122/122 TBPOEHC/TBPB 0.3/0.3 7 100/400(100 +300) 85 119/121 TBPB/TBPB 0.3/0.3 8 100/400(100 + 300) 85 126/121DCP/TBPB 0.3/0.3 9 100/400(100 + 300) 85 123/112 TBPB/TBPOTMH 0.3/0.3 10100/400(100 + 300) 85 123/130 TBPB/DCP 0.3/0.3 11 100/400(100 + 300) 90120/120 TBPB/TBPB 0.6/0.6 12 100/400(100 + 300) 95 120/120 TBPB/TBPB0.3/0.3 13 100/400(100 + 300) 85 120/120 TBPB/TBPB 0.24/0.24 COMP. 1100/400(200 + 200) 85 117/117 TPOEHC/TBPB 0.3/0.3 EXAMPLES 2100/400(200 + 200) 85 130/130 DCP/DCP 0.3/0.3 3 100/400(200 + 200) 60119/119 TPOEHC/TBPB 0.3/0.3 4 100/400(100 + 300) 85 117/121 TBPB/TBPB0.3/0.3 5 100/400(100 + 300) 85 128/121 DCP/TBPB 0.3/0.3 6 100/400(100 +300) 85 123/110 TBPB/TBPOTMH 0.3/0.3 7 100/400(100 + 300) 85 123/132TBPB/DCP 0.3/0.3 8 100/400 — 121 TBPB 0.3 9 100/400(200 + 200) 75120/120 TBPB/TBPB 0.3/0.3

TABLE 2 Falling Dimensional Ball Change Gel PS Powder Compressive ImpactRate Fraction Content Content M.W. Strength Value under (wt %) (wt %)(wt %) Ca. (X10⁴) (N/cm²) (cm) Heat (%) EXAMPLES 1 7.2 22.2 0.7 32 40 850.4 2 26.5 21.9 0.8 31 42 95 0.3 3 33.5 20.9 0.7 30 44 95 0.2 4 20.822.1 0.6 29 42 90 0.3 5 5.8 34.5 0.5 28 40 85 0.4 6 38.5 21.3 0.8 30 4885 0.2 7 7.2 25.0 0.5 30 43 85 0.4 8 3.5 22.8 0.5 30 42 85 0.4 9 3.224.4 0.5 34 43 85 0.3 10 3.8 24.2 0.6 24 43 85 0.4 11 19.6 25.0 0.7 2640 90 0.4 12 18 23.3 0.3 32 46 95 0.3 13 17.5 24.6 0.5 32 46 95 0.2COMP. 1 0.3 26.6 0.5 33 36 80 0.8 EXAMPLES 2 0.8 25.0 0.8 24 38 80 0.7 35.6 21.4 1.6 32 38 60 0.8 4 1.6 25.0 0.5 30 38 80 0.7 5 1.8 25.0 0.6 3040 80 0.7 6 2.6 25.0 0.5 36 37 50 0.4 7 1.8 24.4 0.6 22 40 80 0.7 8 1.720.6 1.8 32 — — — 9 7.2 22.2 1.9 32 45 40 0.4

Example 14

Polyethylene-based resin particles were obtained in the same manner asin Example 1 except that linear low-density polyethylene having adifferent melting point from that used in Example 1 (ethylene-butenecopolymer: melt index of 0.7 g/10 min, density of 0.922 g/ml, meltingpoint of 121° C.) was used.

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles was suspendedin the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 8.0 kg of a styrenemonomer (100 parts by weight relative to 100 parts by weight ofpolyethylene) and 24.0 g of TBPB (0.3 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to118° C. and polymerization was carried out for 3 hours.

After the conversion ratio of polymerization of the styrene monomerreached to 90%, a mixed solution containing 24.0 kg of a styrene monomer(300 parts by weight relative to 100 parts by weight of polyethylene)and 72.0 g of TBPB (0.3 parts by weight relative to 100 parts by weightof the styrene monomer) was added at 118° C. over a period of 4 hours toperform polymerization while impregnating the styrene monomer into theinside of polyethylene. Then, by raising to 140° C. and maintained for 2hours, the remaining monomer was forcibly polymerized to reduce itsamount, and then the autoclave was cooled to obtain styrene-modifiedpolyethylene-based resin particles. A pre-expanded resin particles andexpanded molded article were subsequently obtained in the same manner asin Example 1.

The gel fraction in the resin particles thus obtained was 29.8 wt %. Thepolystyrene content in the gel component was 24.2 wt % and the powdercontent in the polymerized slurry was 0.5 wt %. The molecular weight ofthe polystyrene resin component was about 320,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 43 N/cm², and falling ballimpact value as high as 90 cm. The dimensional change rate under heatwas 0.2%.

The ratio between materials, polymerization conditions, powder content,gel fraction, polystyrene content, molecular weight of the polystyreneresin component, falling ball impact value, compressive strength anddimensional change rate under heat of Example 14 are together shown inTable 3.

TABLE 3 PE/PS(1^(st) and 2^(nd)) 100/400 (100/300) PolymerizationTemperature 118/118 (1^(st)/2^(nd)) (° C.) Conversion ratio ofpolymerization (%) 90 Polymerization Initiator (1^(st)/2^(nd)) TBPB/TBPBAmount of Initiator (1^(st)/2^(nd)) (wt %) 0.3/0.3 Gel Fraction (%) 29.8PS Content (wt %) 24.2 Powder Content (wt %) 0.5 M.W. (×10⁴) About 32Falling Ball Impact Value (cm) 90 Compressive Strength (N/cm²) 43Dimensional Change Rate under Heat (%) 0.2

As seen in Table 3, it is understood that an expanded molded articlehaving excellent impact resistance, stiffness, and heat resistance canbe provided even with the polyethylene-based resin having the differentmelting point.

Example 15

40 kg of pure water, 200 g of magnesium pyrophosphate as a dispersant,and 20 g of sodium dodecylbenzenesulfonate were introduced in anautoclave having an internal volume of 100 liter to prepare an aqueousmedium. 8.0 kg of the polyethylene-based resin particles obtained inExample 1 was suspended in the medium and stirred at 150 rpm.

Into the autoclave, a mixed solution containing 3.2 kg of a styrenemonomer (400 parts by weight relative to 100 parts by weight ofpolyethylene) and 9.6 g of DCP (0.3 parts by weight relative to 100parts by weight of the styrene monomer) was added and allowed to standat 60° C. for 60 minutes to impregnate the styrene monomer into thepolyethylene-based resin particles. Then, the temperature was raised to126° C. and polymerization was carried out for 3 hours.

After the conversion ratio of polymerization of the styrene monomerreached to 90%, a mixed solution containing 28.6 kg of a styrene monomer(358 parts by weight relative to 100 parts by weight of polyethylene),0.2 kg of α-methyl styrene (2 parts by weight relative to 100 parts byweight of polyethylene) and 86.4 g of TBPB (0.3 parts by weight relativeto 100 parts by weight of the styrene monomer) was added at 122° C. overa period of 5 hours to perform polymerization while impregnating styreneinto the inside of polyethylene. Then, by raising to 140° C. andmaintained for 2 hours, the remaining monomer was forcibly polymerizedto reduce its amount, and then the autoclave was cooled to obtainstyrene-modified polyethylene-based resin particles. Pre-expanded resinparticles and an expanded molded article were subsequently obtained inthe same manner as in Example 1.

The gel fraction in the resin particles thus obtained was 30.6 wt %. Thepolystyrene content in the gel component was 25.0 wt % and the powdercontent in the polymerized slurry was 0.8 wt %. The molecular weight ofthe polystyrene resin component was about 300,000.

The pre-expanded resin particles had a bulk density of 33 kg/m³. Theexpanded molded article thus obtained had excellent strength, showing adensity of 33 kg/m³, compressive strength of 42 N/cm², and falling ballimpact value as high as 90 cm. The dimensional change rate under heatwas 0.2%.

The ratio between materials, polymerization conditions, powder content,gel fraction, polystyrene content, molecular weight of the polystyreneresin component, falling ball impact value, compressive strength anddimensional change rate under heat of Example 15 are together shown inTable 4.

TABLE 4 PE/PS(1^(st) and 2^(nd)) 100/400 (40/360) PolymerizationTemperature 126/122 (1^(st)/2^(nd)) (° C.) Conversion ratio ofpolymerization (%) 90 Polymerization Initiator (1^(st)/2^(nd)) DCP/TBPBAmount of Initiator (1^(st)/2^(nd)) (wt %) 0.3/0.3 Gel Fraction (%) 30.6PS Content (wt %) 25.0 Powder Content (wt %) 0.8 M.W. (×10⁴)

30 Falling Ball Impact Value (cm) 90 Compressive Strength (N/cm²) 42Dimensional Change Rate under Heat (%) 0.2

As seen in Table 4, it is understood that an expanded molded articlehaving excellent impact resistance, stiffness, and heat resistance canbe provided even with a mixture of two different styrene monomers.

As described hereinbefore, by controlling selection of thepolymerization initiator and the polymerization temperature atimpregnation and polymerization of the styrene monomer into thepolyethylene-based resin particles for polymerization, the expandableparticles of the styrene-modified linear low-density polyethylene-basedresin which control the gel fraction and which satisfy physicalproperties such as impact resistance, stiffness, and heat resistance canbe provided.

1. A method for producing expandable particles of a styrene-modifiedlinear low-density polyethylene-based resin comprising, in the orderrecited: dispersing 100 parts by weight of non-crosslinked linearlow-density polyethylene-based resin particles, 30 to 300 parts byweight of a styrene-based monomer, and 0.1 to 0.9 parts by weight of apolymerization initiator relative to 100 parts by weight of thestyrene-based monomer into a suspension containing a dispersant;impregnating the styrene-based monomer into the low-densitypolyethylene-based resin particles by heating a resultant dispersion atsuch a temperature that polymerization of the styrene-based monomer doesnot substantially take place; performing a first polymerization of thestyrene-based monomer at a temperature of higher than (T−8) ° C. andlower than (T+1) ° C., T ° C. being a melting point of the low-densitypolyethylene-based resin particles; adding a styrene-based monomer and0.1 to 0.9 parts by weight of a polymerization initiator relative to 100parts by weight of the styrene-based monomer after the firstpolymerization has reached a conversion ratio of from 80% to 99.9%, andperforming an impregnation of the styrene-based monomer into thelow-density polyethylene-based resin particles and a secondpolymerization of the styrene-based monomer at a temperature of higherthan (T−15) ° C. and lower than (T+5) ° C., T ° C. being a melting pointof the polyethylene-based resin particles; and impregnating a volatileblowing agent during or after the polymerization; a total amount of thestyrene monomers used in the first and second polymerizations being morethan 300 parts by weight and not more than 1000 parts by weight relativeto 100 parts by weight of the low-density polyethylene-based resinparticles; whereby resin components of the expandable particles containa gel component comprising from 2 to 40 wt% of a graft polymer.
 2. Themethod of claim 1, wherein the second polymerization is performed at atemperature in a range of from higher than (T−8) ° C. to lower than(T+1) ° C.
 3. The method of claim 1, wherein the linear low-densitypolyethylene-based resin particles each have a substantially sphericalshape or a cylindrical shape having an L/D of from 0.6 to 1.6, L being alength of each particle and D being a diameter of each particle, and anaverage particle size of from 0.2 to 1.5 mm.
 4. The method of claim 1,wherein the styrene-based monomer comprises at least one of styrene,a-methylstyrene, vinyltoluene, and chlorostyrene.
 5. The method of claim1, wherein a molecular weight of a polystyrene-based resin component isfrom about 200,000 to 400,000.
 6. The method of claim 1, wherein thenon-crosslinked linear low-density polyethylene-based resin comprises acopolymer of ethylene and an α-olefin.
 7. The method of claim 6, whereinthe α-olefin comprises at least one of 1-butene, 1-pentene, 1-hexene,3,3-dimethyl-1-butene, 4-methyl-1-pentene, 4,4-dimethyl-1-pentene, and1-octene.
 8. The method of claim 1, wherein 0.2 to 0.5 parts by weightof a polymerization initiator relative to 100 parts by weight of thestyrene-based monomer are used.
 9. The method of claim 1, wherein thetotal amount of the styrene monomers used in the first and secondpolymerizations is not more than 900 parts by weight relative to 100parts by weight of the low-density polyethylene-based resin particles.10. The method of claim 1, wherein the gel component comprises from 5 to35 wt % of a graft polymer.
 11. The method of claim 4, wherein thesecond polymerization is performed at a temperature in a range of fromhigher than (T−8) ° C. to lower than (T+1) ° C.; from 0.2 to 0.5 partsby weight of a polymerization initiator relative to 100 parts by weightof the styrene-based monomer are used; the non-crosslinked linearlow-density polyethylene-based resin comprises a copolymer of ethyleneand at least one of 1-butene and 1-hexene; the total amount of thestyrene monomers used in the first and second polymerizations is notmore than 900 parts by weight relative to 100 parts by weight of thelow-density polyethylene-based resin particles; and the gel componentcomprises from 5 to 35 wt % of a graft polymer.
 12. Expandable particlesof a styrene-modified linear low-density polyethylene-based resin,wherein the particles comprise a volatile blowing agent and a baseresin, the base resin comprising more than 300 parts by weight and lessthan 1000 parts by weight of a polystyrene-based resin componentrelative to 100 parts by weight of a non-crosslinked linear low-densitypolyethylene-based resin component, the base resin comprising from 2 to40 wt % of a gel component comprising a graft copolymer of thepolystyrene-based resin component and the low-density polyethylene-basedresin component.
 13. The expandable particles of claim 12, wherein thebase resin comprises not more than 900 parts by weight of apolystyrene-based resin component relative to 100 parts by weight of anon-crosslinked linear low-density polyethylene-based resin component.14. The expandable particles of claim 13, wherein the base resincomprises from 5 to 35 wt % of a gel component comprising a graftcopolymer of the polystyrene-based resin component and the low-densitypolyethylene-based resin component.
 15. The expandable particles ofclaim 13, wherein a molecular weight of the polystyrene-based resincomponent is from about 200,000 to 400,000.
 16. Expandable particles ofa styrene-modified linear low-density polyethylene-based resin, whereinthe particles are obtained by the method of claim
 1. 17. Pre-expandedparticles having a bulk density of from 20 to 200 kg/m³, obtained bypre-expanding the expandable particles of the styrene-modified linearlow-density polyethylene-based resin of claim
 12. 18. An expanded moldedarticle having a density of from 20 to 200 kg/m³, obtained by expansionmolding of the pre-expanded particles of claim
 17. 19. Pre-expandedparticles having a bulk density of from 20 to 200 kg/m³, obtained bypre-expanding the expandable particles of the styrene-modified linearlow-density polyethylene-based resin of claim
 16. 20. An expanded moldedarticle having a density of 20 to 200 kg/m³, obtained by expansionmolding of the pre-expanded particles of claim 19.